1. Field of the Invention
This invention relates generally to apparatus for reproducing an information signal from a record disc and, more particularly, is directed to apparatus for reproducing a pulse code modulated audio signal from a record disc and using the reproduced signal to control rotation of the disc.
2. Description of the Prior Art
Generally, apparatus of the optical type, electrostatic capacitance type and the like are known in the art for reproducing a pulse code modulated (PCM) audio signal from a record disc, the PCM audio signal generally being recorded on the record disc at a constant angular velocity or at a constant linear velocity. In view of the desire to increase the recording density, recording at a constant linear velocity is preferable and is generally employed. It is therefore to be appreciated that reproduction of the PCM audio signal from the record disc must be performed while the latter is moving at a constant linear velocity.
For controlling rotation of the record disc at the constant linear velocity during the playback mode, it is known to detect the position of a pick-up device by using a potentiometer. Since the number of revolutions is inversely related to the number representing the position of the pick-up device obtained from the potentiometer, the detected output from the potentiometer is fed to a divider circuit to obtain the necessary control information for controlling the disc rotation at the constant linear velocity. However, with such method, the apparatus comprised of the position detector, that is, the potentiometer used for detecting the position of the pick-up device, and the divider circuit, is relatively expensive and complicated in construction.
Accordingly, it has been proposed to control rotation of the record disc at the constant linear velocity by means of the information signal reproduced from the record disc, thereby eliminating the need for a position detector to detect the position of the pick-up device. Before discussing this latter method, it is to be appreciated that, when a PCM audio signal is recorded in the base band, that is, when a PCM audio signal does not utilize a carrier modulation system such as amplitude modulation (AM), frequency modulation (FM) and the like, the PCM audio signal is generally modulated with a run length limited code. Since the PCM audio signal is comprised of a plurality of "0" and "1" bits, a transition interval (T) is defined as the transition between two identical data bits, that is, corresponding to the occurrence of at least one data bit of the "0" or "1" type between two data bits of the other type. In accordance with the run length limited code modulation method, the minimum transition interval T.sub.min that can be used is extended or lengthened to increase the efficiency of the recording, while the maximum transition interval T.sub.max that can be used is shortened to make the self-clocking characteristic of the signal during the playback mode easier to obtain.
In accordance with the aforementioned method of controlling the rotation of the record disc at the constant linear velocity by use of the reproduced signal therefrom, if the record disc is rotated at a constant linear velocity, the maximum transition interval T.sub.max of the reproduced PCM audio signal can be considered as a predetermined reference interval. It is considered that with normal modulation and during normal playback, the reproduced output signal will not include a transition interval greater than the maximum transition interval T.sub.max, and therefore, the occurrence of an interval which is twice the maximum transition interval T.sub.max will certainly not be generated during normal playback. In this regard, a frame synchronizing signal having a length which is exactly twice that of the maximum transition interval T.sub.max is added to each frame of data of the PCM audio signal. If it is considered that this frame synchronizing signal appears during each frame of data of the PCM audio signal without fail, any deviation of the maximum transition interval T.sub.max from a predetermined reference interval can be easily detected, and rotation of the record disc can be controlled so as to cancel any such deviation and thereby control rotation of the record disc at the constant linear velocity.
It is to be appreciated that, in order to detect any deviation of the maximum (or minimum) transition interval from the predetermined reference interval, it is necessary to detect the length of each of the transition intervals of the PCM audio signal. As one method of detecting the length of each transition interval, it has been proposed to generate a clock pulse signal with a constant frequency higher than the bit frequency of the reproduced PCM audio signal, and then count the number of clock pulses in the maximum transition interval to detect the length thereof. Since the number of clock pulses contained in the maximum transition interval when the record disc is rotated at the constant linear velocity is equal to a predetermined value, rotation of the disc is controlled to make the number of clock pulses contained in the detected maximum transition interval equal to the predetermined value.
Since the frame synchronizing signal has a length in each frame that is twice the maximum transition interval, it is generally considered sufficient to detect such maximum transition interval during each frame period that is, the length of time required to reproduce one frame of data at the constant linear velocity. However, when rotation of the record disc occurs at a velocity which is slower than the constant linear velocity, the period of the frame synchronizing signal reproduced from the record disc becomes longer than one frame interval, that is to say, the frame synchronizing signal is not reproduced during the length of time corresponding to one frame period at the constant linear velocity, so that the maximum transition interval cannot be detected over several frame periods. At such time, if the reproduced audio signal from the record disc includes a continuous pianissimo segment, for example, during a classical music segment, and the minimum transition interval T.sub.min is continuous, since the maximum transition interval is detected each frame period, the apparatus may detect the minimum transition interval as the maximum transition interval T.sub.max. In this situation, rotation of the record disc may be locked to a linear velocity lower than the aforementioned constant linear velocity.